Answer: 27 electrons
copper has 29 protons in its nucleus. atoms can only lose or gain electrons. so because copper has 29 protons and will always have 29 protons, and the Cu2+ ion has a positive charge, there are more protons than electrons.
that means there should be 2 less electrons than protons. that leaves us with 29 protons and 27 electrons.
hope this helps :)
